Parallel hybrid vehicles include both a combustion engine and an electric motor that can be selectively coupled to a gearbox or transmission of the vehicle and to the driven wheels. FIG. 1 is a schematic illustration of an exemplary parallel hybrid vehicle 10 known in the art. The parallel hybrid vehicle 10 includes a pair of driven wheels 12, 14 connected via axles 16, 18 to a differential 20, and a pair of non-driven wheels 22, 24 mounted on axles 26, 28. Depending on the particular vehicle 10, the driven wheels 12, 14 may be either the front wheels or the rear wheels of the vehicle 10. An internal combustion engine 30 has an output shaft 32 connected to a friction clutch 34 at one end of a transmission or gearbox 36. A transmission shaft and internal gearing 38 of the transmission 36 connect the friction clutch 34 to the differential 20 by a drive shaft 42. The electrical components of the drive mechanism for the parallel hybrid vehicle 10 include an electric motor 44 connected to the transmission shaft 38 to provide power to the driven wheels 12, 14 along with or as an alternative to the internal combustion engine 30, and also connected to an inverter 46 the transfers electrical power between the electric motor 44 and a battery 48.
In the parallel hybrid vehicle 10 of FIG. 1, the combustion engine 30 and the electric motor 44 can be selectively coupled to the gearbox 38 and then to the driven wheels 12, 14 by the friction clutch 34. The friction clutch 34 is operated according to the driving situation of the vehicle 10 to utilize the internal combustion engine 30 and the electric motor 44 in different ways. For example, when driving in urban areas, the friction clutch 34 may be closed or engaged to connect the internal combustion engine 30 to the transmission shaft 38 so that power from the engine 30 is delivered to the drive shaft 42. At the same time, the motor 44 may be controlled to provide additional power to the drive shaft 42, or to use the power supplied by the electric engine 30 to recharge the battery 48. In other urban situations, the friction clutch 34 may be opened to disconnect the engine 30 from the transmission shaft 38, and power from the battery 48 is used by the motor 44 to drive the vehicle 10. During that situation, the engine 30 may be completely stopped to conserve gasoline. During acceleration of the vehicle 10, the friction clutch 34 may be re-engaged for more responsive acceleration using power from both the engine 30 and the motor 44. In contrast, the friction clutch 34 may be opened to disconnect the engine 30 from the transmission shaft 38 during deceleration so that the motor 44 can efficiently recharge the battery 48 without power loss due to engine friction.
Under the presently known arrangements of the friction clutch 34 in parallel hybrid vehicles 10, the friction clutch 34 in its closed position locks the output shaft 32 and the transmission shaft 38 for rotation together in either direction. In the open position of the friction clutch 34, the output shaft 32 and the transmission shaft 38 are free to rotate relative to each other in either direction. This arrangement can cause inefficiencies in operation of the parallel hybrid vehicles 10. When the friction clutch 34 is engaged for driving the vehicle 10 under the power of the engine 30 in urban areas or while the vehicle 10 is accelerating under the combined power of the engine 30 and the electric motor 44, easing off the gas and slowing the engine 30 can cause rotating losses as the output shaft 32 slows and the transmission shaft 38 correspondingly slows due to the connection provided by the friction clutch 34, unless the friction clutch 34 is actuated to open and disconnect the engine 30 from the transmission 36. If the friction clutch 34 remains closed, the engine rotating losses will be incurred, which may be desired in vehicles 10 where engine breaking is desirable. If the friction clutch 34 is actuated to disconnect the engine 30, the engine rotating losses may be avoided, but open friction clutch rotating losses remain because the relatively large surface area of the facing clutch plate is subjected oil shear resulting in viscous drag. Additionally, the friction clutch 34 must be reclosed when the engine 30 is called upon to provide power to the driven wheels 12, 14. The former option, i.e. leaving the friction clutch 34 closed, may negate the efficiencies sought to be achieved by the hybrid vehicle 10. The latter option, i.e., opening and closing the friction clutch 34, still may negate the efficiencies sought to be achieved due to the viscous drag and corresponding open clutch rotating losses, as well as increase the duty cycle for the actuation mechanism of the friction clutch 34, thereby potentially shortening the lift cycle of the friction clutch 34. In view of this, a need exists for an improved strategy for switching between the power sources of a parallel hybrid vehicle that may further increase the energy efficiency of the hybrid vehicle without unnecessarily increasing rotating losses and the wear and tear on the components in the transmission system of the vehicle.